In say a PLC5 system, You can have 2 seperate racks using 1 slot addressing with Complimentary I/O. Both racks will have the same Rack address but one rack will have input cards and the other will have output cards. Or any combination of input and output cards as long as they are oposite.
Most of the time its used because there are no more Rack address available and more I/O is needed.
No reason to be worried about it but for sure there is no reason to use it unless you have mega I/O and need the available address.